Apparatuses of this type are used, for example, in automatic samplers. Samplers suited for the highly precise taking of liquid samples are available from the assignee, Eudress+Hauser for instance in the form of the ‘ASP-Station 2000’ sampler, wherein time, quantity, and flowrate proportional samples are taken using vacuum equipment. As already indicated, a metering system, utilizing, among other things, a metering container with a lid, serves for the highly precise sample collection.
In the ASP-Station 2000 sampler, the sample taking occurs in four steps: At the beginning of a sample taking, the outlet hose of the metering system is pneumatically clamped to close it, and a pump, in this instance a diaphragm pump, blows through the metering container to clear the suction line for the sample. In a second step, sample is sucked in, until two long conductivity probes, which are secured on the lid of the metering container, respond. Then, the desired sample volume is retained, and excess sample flows back to the location where the sample was taken. In a last step, the constricted outlet hose is opened, and the sample is drained into the appropriate sample bottle. The sample bottles are stored cooled in the ASP-Station 2000 sampler, for the purpose of sample preservation.
In the case of the ASP-Station 2000 sampler, three conductivity probes of different length are secured on the lid of the metering container. The two longer conductivity probes come into contact with the sample medium at the end of the suction phase. Via a current flowing between the conductivity probes, the desired amount of filling of the metering container is detected. The suction procedure is ended, as soon as a current flows between the two conductivity probes.
If conductive substrate deposits in the region of the container lid—thus, if an undesired fouling occurs in the region of the container lid—then both a disturbance current flows between one of the measuring electrodes and the short electrode, and, also, current flows between the two long measuring electrodes. The result of this combination of currents is an immediate safety shutdown of the metering system, with issuance of a corresponding report (Error—Conductivity 2 and/or Error—Conductivity 1) to the operating personnel.
Problematic in the case of the known solution is that, especially in the case of sample media of high conductivity, conductive fouling of the container lid can occur, so that, as a result, a conductive path arises between the electrodes, even only after a relatively short time of operation of the metering system. Now, the resistance of the conductive fouling is, usually, a multiple less than the resistance of the sample medium itself. Thus, with the known solution, as soon as a conductive fouling occurs in the region of the lid of the container, the functioning of the conductive measuring system as a conductive switch is no longer reliable. Consequently, the metering system is promptly shut down and can only be restarted, after the accretions on the lid of the container have been removed. Since this clearing work sometimes has to be repeated in very short cycles, the known sampler can be used for certain sample media only with relatively high maintenance effort. A high maintenance effort is, understandably, undesired.
The following can be concluded: The solution known from the state of the art is not able to recognize the desired fill level of a medium in a container, once conductive accretions have formed on the lid of the container. The metering system is, consequently, shut down, even though this is not really necessary in a number of cases, since the fouling is actually still at a manageable level.
Indeed, there is yet a short, safety electrode arranged on the lid of the metering container of the ASP-Station. However, this safety electrode serves solely for switching off for reasons of safety. If no switching off occurs on the basis of the two long measuring electrodes (e.g. due to an electronics malfunction or to the fact that the measuring electrodes have been fouled by non-conductive material, e.g. fat), then, upon the reaching of a second, predefined fill level, the short safety electrode becomes contacted by the sample medium; this leads, promptly, to a safety shut down, with the accompanying issuance of a report. On the basis of this switching off for reasons of safety, an incursion of the sample medium into the pneumatic control and the vacuum pump is effectively prevented.